The present invention relates to DC/DC converters, particularly for use in battery powered electronic equipment.
For battery powered electronic devices, and for radio transmitting devices such as GSM mobile telephones in particular, using a low voltage supply can present difficulties when running radio frequency (RF) and power amplifier (PA) circuitry. It is desirable to use low voltage batteries in such devices, so that the physical size can be kept small and light in weight. Therefore, DC to DC converters are used in order to boost the supply voltage for the device from the battery voltage to a desired level. Typically DC to DC converters run on a switching frequency, for example 400 KHZ, and it is necessary to filter the output of the converter in order to remove any trace of this switching signal. This makes this kind of solution difficult to implement.
One solution is to run the DC to DC converter only during periods that the power amplifier or RF circuitry is not active, and using a storage device, such as a capacitor, to store charge for supply to PA and RF circuitry during periods of activity.
One such solution is shown in FIG. 1 of the accompanying drawings, and a detailed discussion of this type of circuitry can be found in European Patent Application No. EP0654911. In FIG. 1, the DC to DC converter circuit 1 includes an input node 4 and an output node 6. The input node 4 is connected to a DC voltage Vsupply from a DC source 2. The output node 6 is connected to supply an output voltage Vout to a power amplifier 3. The circuit 1 comprises a first capacitor C1 connected between the input node 4 and ground, and an inductor L1 and a diode D1 connected in series between the input node 4 and the output node 6. A transistor T1 is connected between an intermediate node 5, between the inductor L1 and diode D1, and ground. The diode D1 is connected in forward bias between the intermediate node 5 and the output node 6. A control circuit 8 is connected to receive a reference voltage Vref, which indicates the desired level of Vout, and the output voltage Vout and is operable to control the switching operation of the transistor T1. The converter circuit 1 is generally known as a switched mode power supply, and is described, for example, in xe2x80x9cAdvanced Linear European Seminars for Precision Design of the 90""sxe2x80x9d, published by Texas Instruments UK, 1990 page 59 to 82.
A second capacitor C2 is connected between the output node 6 and ground, such that during inoperative times of the power amplifier, the capacitor C2 is charged to the required PA operating voltage. When the operating voltage is required by the power amplifier 3 it can be supplied without the need for switching of the supply. This removes the need for complex filtering of the output voltage Vout.
However, the problem with this kind of solution is that it requires a very large capacitor C2 which consumes a large amount of space and is expensive.
Accordingly, a DC converter circuit embodying the present invention includes an output capacitor which is connected between the output node and the input node of the device.